The interactive Expression

The interactive expression in the zap-to-char command looks like this:

(interactive "*p\ncZap to char: ")

The part within quotation marks, "*p\ncZap to char: ", specifies three different things. First, and most simply, the asterisk, `*', causes an error to be signalled if the buffer is read-only. This means that if you try zap-to-char in a read-only buffer you will not be able to remove text, and you will receive a message that says "Buffer is read-only"; your terminal may beep at you as well.

The second part of "*p\ncZap to char: " is the `p'. This part is ended by a newline, `\n'. The `p' means that the first argument to the function will be passed the value of a `processed prefix'. The prefix argument is passed by typing C-u and a number, or M- and a number. If the function is called interactively without a prefix, 1 is passed to this argument.

The third part of "*p\ncZap to char: " is `cZap to char: '. In this part, the lower case `c' indicates that interactive expects a prompt and that the argument will be a character. The prompt follows the `c' and is the string `Zap to char: ' (with a space after the colon to make it look good).

What all this does is prepare the arguments to zap-to-char so they are of the right type, and give the user a prompt.

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